TECHNICAL FIELD

[0001] The present invention relates to a method of treating surfaces with photocatalytic treatment substance. The invention relates especially to applying photocatalytic coating by means of spraying. In more detail the invention relates to a method according to the preamble of claim 1.

BACKGROUND

[0002] The term photocatalysis means a chemical reaction caused by light. A photocatalyst is a causes a reaction without taking part in it itself, and a photocatalytic coating is a substance containing photocatalyst. In photocatalysis light initiates a chemical reaction in which the reactive oxygen compounds formed from oxygen and water vapour react with organic matter, such as microbes, on the surface and destroy them. The substance necessary for the reaction, acting as the catalyst, is a semiconductor material, such as titanium dioxide. The method is commonly used for remaining various harmful substances from water and air. Together with light, the catalyst coating destroys virii, bacteria, mould organisms and other volatile organic compounds. The sprayable, photocatalytic materials of the new generation are activated by visible light. The titanium dioxide used in photocatalytic coatings is a non-poisonous and safe material. Usually the material used for surface treatment is water containing small amounts of titanium dioxide and possibly silicon oxide or other additives. The amount of the active substance is small, about 1 percent by weight of the material to be applied, and the amount of any additives is even smaller than that. Thus the consistency of the material to be applied is water-like. Applying material with only a small amount of active substance to surfaces is difficult, as excessive wetting must often be avoided, the amount of the active substance on the surface must be sufficient and the coverage of the coating must also be sufficiently homogenous.

[0003] Publications JP 2019010643 and JP 2017213566 describe applying a photocatalyst to surfaces. In publication JP 2020040059 the photocatalyst is fine-grained titanium dioxide. SUMMARY

[0004] The aim of the invention is to provide a new method of applying especially for applying nanoparticle-sized photocatalytic coatings.

[0005] The invention is characterized by what is disclosed in the characterizing part of the independent claim.

[0006] According to one embodiment of the invention in the method of applying a photocatalyst coating a treatment substance is formed, the material comprising water, photocatalyst in nanoparticle form and stabilizing agent. The treatment substance is applied onto the surface by means of a droplet-forming nozzle and a temperature gradient is formed into the coating mixture for aggregating the nanoparticles.

[0007] According to a second embodiment of the invention the photocatalytic coating is applied by means of an air-atomizing nozzle, the air for the nozzle is produced by means of a compressor in which the air heats as it is compressed, and the temperature of the nozzle is kept higher than ambient temperature. [0008] According to a second embodiment of the invention the photocatalytic coating is applied by means an ultrasound nozzle. The ultrasound nozzle can consist of a piezoelectric vibration generator or the ultrasound can be provided to the pressurized flowing coating material by means of the geometry of the nozzle, whereby no external vibration generator is needed.

EMBODIMENTS

[0009] DEFINITIONS

[0010] In this context the expression “ambient temperature” means the temperature of the working environment and more specifically a common room temperature of 20°C.

[0011] The properties of the treatment substance containing photocatalyst in nanoparticle form closely correspond to those of water. Thus it is essentially different from conventional surface treatment materials, such as varnishes and paints. Compared with these, the viscosity of water is low and the amount of solids nearly non-existent. The coating is also practically invisible, so producing a homogenous coverage can be challenging. Air-atomizing or ultrasound-atomizing nozzles can be used for applying the coating. In an air-atomizing nozzle the treatment substance is atomized into droplet form by means of a fast-flowing airflow. This airflow is produced by means of a compressor. Efficient atomizing is achieved when air and treatment material are pushed through the nozzle opening of the nozzle. In an air-atomizing nozzle airflow and the amount of introduced substance material can be separately adjusted. This is necessary for effective applying of nanoparticle-containing photocatalyst coating, because the required amount of treatment material is small and a fast airflow requiring a sufficient air volume flow is needed for efficient forming of droplets. In an ultrasound-atomizing nozzle the treatment material is atomized into droplet form by means of cavitation caused by ultrasound. A temperature gradient advantageous for droplet forming and the desired phase transitions is achieved especially when the cavitation is inertial cavitation.

[0012] The photocatalyst in the form of nanoparticles is a very efficient substance used for disintegrating biological impurities and organic compounds. Usually the photocatalyst is titanium dioxide, but the treatment substance can comprise other materials as well, such as silicon dioxide. The active substance is mixed and stabilized in water and the total solids consistency of the treatment agent is about 1 % by weight. The stabilization is effected by means of suitable polymers encapsulating the nanoparticles. The stabilization agent can be, for example, water-soluble thermoplastic polymer or a non soluble polymer colloid (such as latex). The application is carried out by means of an air- atomizing nozzle, especially a high-pressure air-atomizing nozzle. The compressed air is produced by means of a compressor and the compressed air is introduced into a nozzle into which treatment substance is simultaneously introduced. The treatment substance and air are mixed in the nozzle and they are spread from the nozzle opening as a fine mist onto the surface to be treated. When air is compressed in the compressor, its temperature rises. The increase of temperature is utilized during application of the coating. The heat energy contained by the hot air has an effect on the energy balance in the nozzle and the treatment substance being expelled from the nozzle. In practice it has been noticed that treatment mist produced by means of air heated in the compressor produces a faster drying and more homogenous coating. Thus it can be contemplated that a high air pressure and the high feed and mixing speed in nozzle caused thereby in combination with elevated temperature produce a very fine and quickly drying treatment substance mist. An ultrasound-atomizing nozzle can also be used for application of the substance by utilizing heterogenous inertial cavitation. The advantage of heterogenous inertial cavitation is that the temperature gradient caused by cavitation is localized in the interface between the nanoparticle and water, whereby the phase transition of the polymer used for the encapsulation is possible without considerably increasing the temperature of the whole of treatment agent. Thus a local temperature gradient can be considerably higher than the temperature of the surrounding water.

[0013] During application of the coating mixture the purpose is to form a temperature gradient into the mixture to be applied. The temperature change allows having an effect on the formation of coating at the water/stabilizer/catalyst interfaces. This allows utilization of heat and microdroplets for producing an even coating. Heat can further be utilized for encapsulating the nanoparticles and the phase transition of the polymers used for stabilizing, whereby the nanoparticles are aggregated already in the droplet formed by the nozzle. The source of heat is either the thermodynamics caused by the pressurizing of the colloid-air mixture and/or the strong local temperature gradient caused by the heterogenous inertial cavitation caused by the ultrasound nozzle. The advantage of the inertial cavitation of the ultrasound nozzle is that it allows a very big gradient and the phase transition of the polymers used for encapsulating is nearly immediate. Thus the heterogenous inertial cavitation takes place in the vicinity of the large number of various interfaces contained by the nanoparticle colloid. Cavitation can also be caused by using ultrasound nozzles with a separate piezoelectric vibration generator or it can be caused by means of special geometry of the nozzle. As can be seen from the above, the temperature gradient is such that it increases the temperature of the coating mixture. On the surface to be coated the temperature of the coating will be aligned with the temperature of the surface.

[0014] In the method a mist is formed of the water-based colloid, wherein the droplet size of the mist is in practice between 10 microns and 20 microns. Thereby the droplets are formed from the colloid containing the nanoparticles. The mist is aimed at the surface to be coated. The temperature of the mist in the nozzle, more specifically the temperature gradient aimed at the coating, is set so that the phase of the above-mentioned polymer film changes so that the nanoparticles inside the capsule are aggregated inside the droplet released from the nozzle before colliding with the surface to be coated. The suitable temperature depends on the colloid and other properties of the mixture. [0015] The colloidicity of the nanoparticles and the colloid film changes according to the temperature, which allows changing the behaviour of the droplet by adjusting the temperature. Due to the nanoparticles aggregating inside the droplet, hardly any nanoparticle dust is released into the surroundings. On the other hand, the composition of the film of the droplet must be heated so that the droplets disintegrate in a controlled manner on the surface to be coated and to apply the reacting material on the surface. By combining the nanoparticle coating, stabilizing agent, such as the above-mentioned colloid, and controlled heating a novel method of applying and producing photocatalytic coating is provided, the coating produced by means of which is long lasting, efficient and with low consumption of catalyst.

[0016] One of the commercially available coating apparatuses FinishPro HVLP 9.5 Procomp Series Sprayer. The manufacturer of the apparatus is GRACO. This apparatus has a separate compressor for producing compressed air and a separate inlet for the treatment material. The intended use of the apparatus is applying viscous treatment materials. Its compressor will produce a pressure of 25 psi (172 N/m ² ) and the pressure can be finely adjusted by means of a pressure regulator and a gauge. The recommended operation pressure is 10 psi (69 N/m ² ). This apparatus allows forming a very fine spray/mist and the warm air will dry the thin coating layer very quickly.

[0017] Temperatures of the apparatus have been measured during operation. The temperature of the compressed air hose and the handle of the nozzle was about 30 degrees Celsius, the surface temperature of the nozzle about 32 degrees Celsius and the mist exiting the nozzle 25 degrees Celsius, varying between 24 to 28 degrees Celsius. The temperature of the compressor was about 90 to 115 degrees Celsius. Coating near the nozzle about 15 degrees Celsius (energy is probably spent on evaporation). These are surface measurements from the external surfaces of the apparatus, i.e. inside the hose and other flow channels the air is probably warmer. On the basis of the above-mentioned it is conceivable that together with the decrease of pressure in the nozzle the energy of the warm air effectively evaporates moisture from the treatment material. In order to get this advantage the temperature of the compressed air must be increased so as to be higher than the ambient temperature, i.e. over 20°C. However, as the treatment substance must not evaporate, the air temperature should not be over 100°C. It is also conceivable that the treatment substance be heated, but its effect is small, as the mass flow of the treatment substance is small compared to the mass flow of air. From the measurements it can be observed that advantageous effects can be achieved if the external temperature of the nozzle is over 20°C. Thereby the temperature of the mixed air and treatment material is higher than that. On the other hand, the nozzle can also heat up due to the flow, which then heats the material to be sprayed. Thus the temperature of the nozzle is an indication of operating in the correct temperature range.

[0018] It is naturally conceivable that compressed air is heated by means of a separate heater, but such will increase the price of the apparatus.

[0019] One reason for the better operation in higher temperatures can be explained by the polymer part of the nanocolloid. The sprayed photocatalytic surface contains a binding material, i.e. a matrix, consisting of a water-soluble thermoplastic polymer or a non-soluble polymer colloid (such as latex). When the temperature of the colloid is increased to the glass transition temperature (Tg) or even to its melting temperature (Tm), it softens/melts, and when it collides with the surface to be applied, it is more easily shaped to follow the contours of the surface. A matrix, better shaped to follow the contours of the surface, creates a better adhesion with the surface, as the physical-chemical forces that bind surfaces to each other are directly dependent on their contact area. On the other hand, the increase of temperature lowers both the surface tension and the viscosity of water, whereby a smaller droplet size is possible with a certain pressure. The same result can be achieved by using a chemical surfactant compound. A smaller droplet size allows faster evaporation of water as the temperature and evaporation area increase, whereby the colloid is highly concentrated as it collides with the surface, thereby avoiding so-called dripping, the surface dries faster and it becomes more homogenous.

INDUSTRIAL APPLICABILITY

[0020] The invention can be used for treating surfaces, especially for protecting surfaces from microbes, and for microbiological cleaning of the treated space through function of the coating.